I constructed a second commercial port to avoid traffic. Both ports are full of employees well paid. However, since the construction of the second port the merchant boats are getting no service any more.
The biggest benefit of a second port is that you can import more goods quickly - each boat will only import 500 of each type of goods, per visit. If your industries use imported goods, then you can run out of input goods before the next boat comes. A second port means you won't run out of input goods for your factories. If you have multiple factories using the same input goods, you definitely need more docks to keep them supplied.
A second port also smooths your income out - instead of shipping $300,000 worth of goods every six months or so, you ship $150,000 of goods every three months or so, on average. (Sometimes the two cargo ships arrive at the same time, and you lose that benefit until they fall out of sync. Also, factories usually ship their goods to the closest dock, so one dock will probably get more goods than the other.)
If you're having problems with dockworkers not servicing your docks, make sure that you have enough garages so your dockworkers can always find a car to get to the docks quickly. I've seen plenty of Tropicans drive out to some remote farm, bar, church, clinic, or house, and then take a very long time walking back because there wasn't a nearby garage; this can make your dockworkers miss the boat.